Waveguide grating with spatial variation of optical phase

ABSTRACT

An optical waveguide is disclosed. The optical waveguide includes a plate of transparent material comprising opposed first and second surfaces for guiding an optical beam between the surfaces by at least one of reflection or diffraction. A diffraction grating is disposed at the first surface for spreading the optical beam by diffracting portions thereof into a non-zero diffraction order to propagate inside the plate. The first diffraction grating includes an array of parallel grooves structured to provide a spatial variation of optical phase of the portions of the optical beam diffracted by the first diffraction grating into the non-zero diffraction order.

REFERENCE TO RELATED APPLICATION

The present invention is a continuation of U.S. patent application Ser.No. 16/139,820 filed on Sep. 24, 2018 and incorporated herein byreference in its entirety.

TECHNICAL FIELD

The present disclosure relates to optical components and modules, and inparticular to optical waveguide based components and modules usable indisplay systems.

BACKGROUND

Head-mounted displays (HMDs), near-eye displays, and other kinds ofwearable display systems can be used to provide virtual scenery to auser, or to augment a real scenery with additional information orvirtual objects. The virtual or augmented scenery can bethree-dimensional (3D) to enhance the experience and to match virtualobjects to the real 3D scenery observed by the user. In some displaysystems, a head and/or eye position and orientation of the user aretracked in real time, and the displayed scenery is dynamically adjusteddepending on the user's head orientation and gaze direction, to provideexperience of immersion into a simulated or augmented 3D environment.

It is desirable to reduce size and weight of a wearable display.Lightweight and compact near-eye displays reduce the strain on user'shead and neck, and are generally more comfortable to wear. Typically, anoptics block is one of heaviest modules of the display. Compact planaroptical components, such as waveguides, gratings, Fresnel lenses, etc.,can be used to reduce size and weight of an optics block. However,compact planar optics may be prone to optical distortions andaberrations.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1A is a plan view of an optical waveguide for pupil expansion in anear-eye display;

FIG. 1B is a k-vector plot corresponding to beam propagation in theoptical waveguide of FIG. 1A;

FIG. 2A is a magnified view of the optical waveguide of FIG. 1A showingmultiple paths of an optical beam;

FIG. 2B is a k-vector plot corresponding to multi-path beam propagationof FIG. 2A;

FIG. 3 is a magnified view of an optical waveguide structured to providea spatial variation of optical phase by meandering the grating grooves;

FIG. 4 is a magnified view of an optical waveguide structured to providea spatial variation of optical phase by spatially varying fill factor ofthe diffraction grating;

FIG. 5 is a plan view of an optical waveguide for a near-eye displayshowing a simulated ray-traced expanded optical beam;

FIG. 6 is a plan view of an optical waveguide with meanderingdiffraction grating grooves with offset diffraction gratings;

FIG. 7A a plan view of an optical waveguide with meandering diffractiongrating grooves with superimposed diffraction gratings;

FIG. 7B is a k-vector plot corresponding to normal beam propagation inthe optical waveguide of FIG. 7A;

FIG. 8 is a flow chart of a method r reducing a spatial variation ofthroughput of an optical waveguide according to the present disclosure;

FIG. 9A is an isometric view of an eyeglasses form factor near-eye AR/VRdisplay incorporating an optical waveguide of the present disclosure;

FIG. 9B is a side cross-sectional view of the display of FIG. 9A; and

FIG. 10 is an isometric view of a head-mounted display (HMD)incorporating an optical waveguide of the present disclosure.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated.

An imaging waveguide of a near-eye optical display carries a beam ofimage light from a projector to an eye of a user. The beam of imagelight propagates in the waveguide via multiple reflections from innerwaveguide surfaces and diffraction on grating structures of thewaveguide. There may exist multiple optical paths for propagating beamexiting the waveguide at a particular location of an eyebox. Portions ofthe beam propagating along these multiple optical paths may interferewith one another causing brightness and color variation of the observedimage. In accordance with the disclosure, a diffraction grating of thewaveguide may be configured to provide a spatial variation of opticalphase of the portions of the optical beam diffracted by the diffractiongrating, destroying or at least reducing the unwanted interference andthereby reducing undesired brightness and color variations of theobserved image.

In accordance with the present disclosure, there is provided an opticalwaveguide comprising a plate of transparent material comprising opposedfirst and second surfaces for guiding an optical beam therebetween by atleast one of reflection or diffraction. A first diffraction grating isdisposed at the first surface for spreading the optical beam bydiffracting portions thereof into a non-zero diffraction order topropagate inside the plate. The first diffraction grating comprises anarray of grooves running parallel to one another and structured toprovide a spatial variation of optical phase of the portions of theoptical beam diffracted by the first diffraction grating into thenon-zero diffraction order.

To provide the spatial variation of optical phase of the portions of theoptical beam diffracted by the first diffraction grating into thenon-zero diffraction order, the grooves of the first diffraction gratingmay be made to meander in a plane of the first diffraction grating. Thegrooves may be spaced apart at a first grating pitch, such that anamplitude of meander of the grooves is no greater than the first gratingpitch. The amplitude of meander may be spatially varying, e.g. in aperiodic or a pseudo-random pattern. A period of the periodic patternmay be e.g. greater than 2 mm. In some embodiments, the period of theperiodic pattern is greater than 2 mm and an amplitude of meander of thegrooves is no greater than 500 nm. In some embodiments, the spatialvariation of optical phase is no greater than 27c. The first diffractiongrating may include a surface-relief diffraction grating. The grooves ofthe surface relief diffraction grating may have a spatially varying fillfactor for providing the spatial variation of optical phase of theportions of the optical beam diffracted by the first diffractiongrating.

A second diffraction grating may be provided at the second surface foroutputting the optical beam by diffracting portions thereof to propagateout of the plate. The second diffraction grating may be laterally offsetfrom the first diffraction grating in a direction of diffraction of theportions of the optical beam on the first diffraction grating. Inembodiments where the second diffraction grating is disposed oppositethe first diffraction grating, the second diffraction grating mayinclude an array of grooves running parallel to one another andstructured to provide a spatial variation of optical phase of theportions of the optical beam diffracted by the second diffractiongrating. The optical waveguide may include an input coupler for couplingthe optical beam into the optical waveguide.

In accordance with an aspect of the present disclosure, there is furtherprovided an optics block for a near-eye display, the optics blockcomprising a waveguide described above and an image source opticallycoupled to the input coupler for providing the optical beam thereto. Inoperation, the optical beam carries an image to be displayed by thenear-eye display.

In accordance with an aspect of the present disclosure, there is furtherprovided a method for reducing a spatial variation of throughput of anoptical waveguide comprising a plate of transparent material havingopposed first and second surfaces for guiding an optical beamtherebetween by at least one of reflection or diffraction. The methodmay include providing a first diffraction grating at the first surfaceof the plate, for spreading the optical beam by diffracting portionsthereof into a non-zero diffraction order to propagate inside the plate.The first diffraction grating may include an array of grooves runningparallel to one another and structured to provide a spatial variation ofoptical phase of the portions of the optical beam diffracted by thefirst diffraction grating into the non-zero diffraction order. Thegrooves of the first diffraction grating may be meandering in a plane ofthe first diffraction grating to provide the spatial variation ofoptical phase of the portions of the optical beam diffracted by thefirst diffraction grating. The grooves may have a spatially varying dutycycle for providing the spatial variation of optical phase of theportions of the optical beam diffracted by the first diffractiongrating.

Referring now to FIG. 1A, an optical waveguide 100 includes a plate 110of transparent material comprising opposed first 101 and second 102surfaces for guiding an optical beam 104 between the first 101 andsecond 102 surfaces by total internal reflection (TIR) inside the plate110 in a zigzag pattern spanning between the first 101 and second 102surfaces. The zigzag pattern is not seen as such, as it is viewed fromtop in FIG. 1A. The optical beam 104 may be generated by an image source106. The image source 106 may generate an image in angular domain, whichis carried by the diverging optical beam 104. Herein, the term “image inangular domain” means image, in which different elements of the image(pixels) are represented by angles of corresponding rays of the opticalbeam 104. The optical beam 104 is coupled into the optical waveguide 100by an input coupler, e.g. a diffraction grating coupler 108. The plate110 may be made of glass, quartz, sapphire, etc., or any other materialsufficiently transparent for transmitting through at least a portion ofincoming light, at wavelength(s) of the optical beam 104. The first 101and second 102 surfaces are made to be parallel to each other; however,manufacturing tolerances may lead to wedging or waving of the plate 110,which may lead to undesired brightness/color variation, as explainedbelow.

A first diffraction grating 121 is disposed at the first surface 101 forspreading the optical beam 104 by diffracting portions 104′ of theoptical beam 104 into a non-zero, e.g. first, diffraction order topropagate inside the plate 110. The diffracted portions 104′ of theoptical beam 104 propagate, via TIR in zigzag pattern, towards a seconddiffraction grating 122, which is laterally offset from the firstdiffraction grating 121 in the direction of diffraction of the portions104′ of the optical beam 104, i.e. downwards in FIG. 1A. The seconddiffraction grating 122 out-couples the portions 104′ via diffraction,at multiple locations represented by circles 112, which together definean eyebox 114, i.e. an area when a good-quality image may be directlyobserved by a user.

To carry the image with as few distortions as possible, the ray anglesin the optical beam 104 need to be preserved for all rays within a fieldof view (FOV) of the display. The diffraction grating coupler 108 andthe first 121 and second 122 diffraction gratings may be configured tofulfill that condition. Referring to FIG. 1B, an in-coupling k-vector138 of the diffraction grating coupler 108, a first k-vector 131 of thefirst diffraction grating 121, and a second k-vector 132 of the seconddiffraction grating 122 are sized and oriented such that the vector sumof these three k-vectors is zero. When this condition is fulfilled, theray angles in the optical beam 104 can be preserved.

Referring to FIG. 2A, the beam spreading function of the firstdiffraction grating 121 is further illustrated. The optical beam 104diffracts on the first diffraction grating 121 at multiple locations201, 202, 203, 204, and 205 producing portions 104′, 104″, and 144propagating downwards in FIG. 2A. The optical paths of a selectedportion 144 will be now considered. The main optical path includes adiffraction at the fifth location 205, and is denoted with a solidarrow. Multiple optical paths are available for the selected portion144. For instance, a first additional optical path 211 is available(shown in dotted line). When propagating along the first additionaloptical path 211, a first optical beam portion 104′ is diffracted at thefirst location 201, then at a sixth location 206, and then at a seventhlocation 207. A second additional optical path 212 is also available(shown in long-dashed line). When propagating along the secondadditional optical path 212, a second optical beam portion 104″ isdiffracted at the second location 202, then at an eighth location 208,and then at a ninth location 209. The first 204′ and second 204″ beamportions interfere with the main portion 144. If the plate 110 (FIG. 1A)were perfectly straight and plano-parallel, the interference would havea same effect across the first diffraction grating 121; however, theplate 110 is rarely perfectly flat and plano-parallel, which causes thethroughout of the waveguide 100 to be spatially variant. The situationis exacerbated by the fact that the optical path lengths of the first211, second 212, and the main optical path of the selected portion 144are all the same, such that a finite spectral bandwidth of the opticalbeam does not reduce the optical interference by much. It is furthernoted that, as evidenced by FIG. 2B, the vector sum of k-vectors is alsozero for the additional optical paths 211 and 212. This happens becausea k-vector 256 corresponding to diffraction at the sixth location 206and a k-vector 257 corresponding to diffraction at the seventh location207 cancel each other, and k-vectors 258 corresponding to diffraction atthe eighth location 208 and 259 corresponding to diffraction at theninth location 209 also cancel each other.

Turning now to FIG. 3, a diffraction grating 321 may be used in place ofthe first diffraction grating 121 of the waveguide 100 of FIG. 1A. Thediffraction grating 321 of FIG. 3 may be a surface relief grating andmay include an array of grooves 302 running parallel to one another andmeandering, i.e. waving, in a plane of the diffraction grating 321 toprovide a spatial variation of optical phase of the portions 104′, 104″,144 of the optical beam 104 diffracted by the diffraction grating 321into the non-zero diffraction order. The spatial variation of theoptical phase suppresses, i.e. averages out, undesired interferenceeffects due to a pseudo-random nature of optical interference with addedpseudo-random phase shifts. At the same time, a reduction of amodulation transfer function (MTF) due to the added random phase isminimal, since for the main portion 144 the randomly varying opticalphase shift is added only once, at the diffraction at the fifth location205 (FIG. 2A), while for the first 211, second 212, and other similaradditional optical paths, the randomly varying optical phase shift isadded three or more times. The amplitude of meandering is greatlyexaggerated in FIG. 3 for the purposes of illustration.

The required amplitude of meander of the grooves 302 is typically verysmall. This is because the variation of optical phase required to reducethe interference-caused throughput non-uniformity of the waveguide 100is quite small, e.g. no greater than 27c. In some embodiments, anamplitude of meander is no greater than a grating pitch of the gratinggrooves 302, i.e. no greater than a distance between neighboring grooves302. This is illustrated schematically by a straight line 307. For asinusoidal meandering, a period of the sinusoidal pattern may be greaterthan 2 mm at a grating pitch of less than one micrometer. In anothernon-limiting example, the period of the sinusoidal pattern is greaterthan 2 mm and an amplitude of meander of the grooves is no greater than500 nm. The meandering does not need to be sinusoidal; any other smoothperiodic or aperiodic, or even a completely random or pseudo-randommeandering pattern may also be used. The amplitude of meander may beconstant or spatially varying for further optimization of image quality.

The spatial variation of optical phase of the portions of the opticalbeam diffracted by the first diffraction grating into the non-zerodiffraction order may be achieved in a variety of ways. In anon-limiting example illustrated in FIG. 4, grooves 402 of the surfacerelief diffraction grating 421 run parallel to one another and have aspatially varying duty cycle, or fill factor, for providing the spatialvariation of optical phase of the portions of the optical beamdiffracted by the diffraction grating 421. The spatially varying fillfactor may be achieved e.g. by varying thickness or width of thediffraction grooves themselves, as represented schematically in FIG. 4by a spatially varying thickness of solid lines representing the gratinggrooves 402.

Turning to FIG. 5, a result of ray-tracing an optical beam 504 in abeam-expanding waveguide 500 is shown. The image scale is shown at thebottom of FIG. 5. The beam-expanding waveguide 500 includes atransparent plano-parallel plate 510 supporting an input coupler 508 forcoupling an optical beam 504 emitted by an image source 506 into thebeam-expanding waveguide 500. A first diffraction grating 521 (shown bysolid lines) is disposed at a front surface of the plate 510. The firstdiffraction grating 521 expands the optical beam 504 by diffractingportions 504′ of the optical beam 504 propagating in the plate 510 in azigzag pattern by TIR from inside the plate 510 towards bottom-leftcorner in FIG. 5. A second diffraction grating 522 (shown by dashedlines) is disposed at a rear surface of the plate 510. The seconddiffraction grating 522 out-couples the portions 504′ of the opticalbeam 504 from the plate 510, as indicated by darker areas 550. Arhomboidal expanding pattern of beam portions 504′ is obtained in thissimulation. Each turn of the optical beam 504 or its portions 504′towards the bottom-left corner in FIG. 5 corresponds to diffraction onthe diffraction grating 521, while straight sections of the opticalbeams directed towards bottom-right corner in FIG. 5 correspond topropagation of the optical beam portions 504′ via TIR on a surface ofthe plate 510 and zero-order diffraction on the first diffractiongrating 521. In such a configuration, each diffraction of the opticalbeam 504 or its portions 504′ by the second diffraction grating 522 canbe followed by a secondary diffraction of the diffracted light by thefirst diffraction grating 521, which may further increase field of viewof the beam-expanding waveguide 500.

Referring now to FIG. 6, an optical waveguide 600 is similar to theoptical waveguide 100 of FIG. 1A. The optical waveguide 600 includes aplate 610 of transparent material having opposed first 601 and second602 surfaces for guiding an optical beam 604 between the first 601 andsecond 602 surfaces by total internal reflection (TIR) from inside theplate 610 in a zigzag pattern. The optical beam 604 may be generated byan image source 606. The image source 606 may generate image in angulardomain. This image is carried by the diverging optical beam 604. Theoptical beam 604 is coupled into the optical waveguide 600 by an inputgrating coupler 608. The plate 610 may be made of glass, quartz,sapphire, etc., or any other material transparent, i.e. transmittingthrough at least a portion of incoming light, at wavelength(s) of theoptical beam 604. A first diffraction grating 621 with meanderinggrooves is similar to the diffraction grating 321 of FIG. 3 in that itprovides the spatial variation of optical phase of the portions of theoptical beam diffracted by the first diffraction grating 621 into thenon-zero diffraction order. A diffraction grating with spatially varyingfill factor or duty cycle, similar to the diffraction grating 421 ofFIG. 4, may also be used. A second diffraction grating 622 is laterallyoffset from the first diffraction grating 621 in a direction ofdiffraction of portions 604′ of the optical beam 604 on the firstdiffraction grating 621. The second diffraction grating 621 may bedisposed on the first 601 or second 602 surface of the plate 610.

In some embodiments, the diffraction gratings may be disposed againstone another on opposed surfaces of the waveguide. FIG. 7A illustratessuch an embodiment. Similarly to the above described optical waveguide600 of FIG. 6, an optical waveguide 700 of FIG. 7A is based on a plate710 of transparent material having opposed first 701 and second 702surfaces for guiding an optical beam 704 between the first 701 andsecond 702 surfaces by TIR and/or diffraction. The optical beam 704 maybe generated by an image source 706 disposed behind or in front of thewaveguide 700. The image source 706 may generate image in angular domainto be carried by the diverging optical beam 704. The optical beam 104 iscoupled into the optical waveguide 600 by an input grating coupler 708.A first diffraction grating 721 with meandering grooves shown in solidlines is similar to the diffraction grating 321 of FIG. 3. A seconddiffraction grating 722 with meandering grooves shown in dotted lines isdisposed opposite the first diffraction grating 721, i.e. on the secondsurface 702 of the plate 710. Grooves of the second diffraction grating722 run parallel to one another and are meandering to provide a spatialvariation of optical phase of the portions of the optical beam 704diffracted by the second diffraction grating 722. An area ofintersection of the first 721 and second 722 diffraction gratingsdefines an eyebox 740. In some embodiments, the grooves of the first 721and/or second 722 diffraction grating may have a varying duty cycle,similar to the grooves 402 of the diffraction grating 421 of FIG. 4.

The k-vector diagram for the above configuration of the input gratingcoupler 108, the first diffraction grating 721, and the seconddiffraction grating 722 is illustrated in FIG. 7B. An in-couplingk-vector 738 is oriented downwards in this example. A pair of firstk-vectors 731 (solid arrows) corresponds to the first diffractiongrating 721, and a pair of second k-vectors 732 (dashed arrows)corresponds to the second diffraction grating 722. The optical beam 704,directed downwards by the input grating coupler 708, is diffracted byboth the first 721 and second 722 diffraction gratings in two differentdirections corresponding to the left and right triangles in FIG. 7B,formed by the k-vectors 738 (common side of the two triangles), 731, and732.

Referring to FIG. 8, a method 800 for reducing a spatial variation ofthroughput of an optical waveguide including a plate of transparentmaterial, such as the plate 110 of FIG. 1A, 510 of FIG. 5, 610 of FIG.6, or 710 of FIG. 7A, includes providing (802) a first diffractiongrating (e.g., the first diffraction grating 121 of FIG. 1A, the firstdiffraction grating 521 of FIG. 5, the first diffraction grating 621 ofFIG. 6, or the first diffraction grating 721 of FIG. 7A) at the firstsurface of the plate, for spreading the optical beam by diffractingportions of the optical beam into a non-zero diffraction order topropagate inside the plate. The first diffraction grating comprises anarray of parallel grooves structured to impart a spatial variation ofoptical phase of the portions of the optical beam diffracted by thefirst diffraction grating into the non-zero diffraction order, asexplained above with reference to FIGS. 1A, 5, 6, and 7A. A seconddiffraction grating may also be provided (804) on the first or secondsurface of the plate. The second diffraction grating may include anarray of parallel grooves, which optionally may also be structured toimpart a spatial variation of optical phase of the portions of theoptical beam diffracted by the first diffraction grating into thenon-zero diffraction order. To provide the spatial variation of opticalphase, the grooves of the diffraction grating(s) may be meandered orwiggled, e.g. in a sinusoidal or a pseudo-random pattern, and/or thethickness of the grooves may be spatially varied. Other structuralvariations of the first and second diffraction gratings may be employed,as well.

Referring to FIGS. 9A and 9B, a near-eye AR/VR display 900 may includewaveguides of the present disclosure, e.g. the waveguide 100 of FIG. 1A,the waveguide 500 of FIG. 5, the waveguide 600 of FIG. 6, or thewaveguide 700 of FIG. 7A, to guide image light to eyeboxes 910 of thenear-eye AR/VR display 900. A body or frame 902 of the near-eye AR/VRdisplay 900 has a form factor of a pair of eyeglasses, as shown. Adisplay unit 904 includes a display assembly 906 (FIG. 9B) whichprovides image light 908 to the eyebox 910, i.e. a geometrical areawhere a good-quality image may be presented to a user's eye 912. Thedisplay assembly 906 may include a separate AR/VR display module foreach eye, or one AR/VR display module for both eyes. For the lattercase, an optical switching device may be coupled to a single electronicdisplay for directing images to the left and right eyes of the user in atime-sequential manner, one frame for left eye and one frame for righteye. The images may be presented fast enough, i.e. with a fast enoughframe rate, that the individual eyes do not notice the flicker andperceive smooth, steady images of surrounding virtual or augmentedscenery.

An electronic display of the display assembly 906 may include, forexample and without limitation, a liquid crystal display (LCD), anorganic light emitting display (OLED), an inorganic light emittingdisplay (ILED), an active-matrix organic light-emitting diode (AMOLED)display, a transparent organic light emitting diode (TOLED) display, aprojector, or a combination thereof. The near-eye AR/VR display 900 mayalso include an eye-tracking system 914 for determining, in real time, agaze direction and/or the vergence angle of the user's eyes 912. Thedetermined gaze direction and vergence angle may also be used forreal-time compensation of visual artifacts dependent on the angle ofview and eye position. Furthermore, the determined vergence and gazeangles may be used for interaction with the user, highlighting objects,bringing objects to the foreground, dynamically creating additionalobjects or pointers, etc. Furthermore, the near-eye AR/VR display 900may include an audio system, such as small speakers or headphones.

Turning now to FIG. 10, an HMD 1000 is an example of an AR/VR wearabledisplay system which encloses user's face, for a greater degree ofimmersion into the AR/VR environment. The HMD 1000 may includewaveguides of the present disclosure, e.g. the waveguide 100 of FIG. 1A,the waveguide 500 of FIG. 5, the waveguide 600 of FIG. 6, or thewaveguide 700 of FIG. 7A, to guide image light to eyeboxes, not shown,of the HMD 1000. The HMD 1000 can present content to the user as a partof an AR/VR system, which may further include a user position andorientation tracking system, an external camera, a gesture recognitionsystem, control means for providing user input and controls to thesystem, and a central console for storing software programs and otherdata for interacting with the user for interacting with the AR/VRenvironment. The function of the HMD 1000 is to augment views of aphysical, real-world environment with computer-generated imagery, and/orto generate entirely virtual 3D imagery. The HMD 1000 may include afront body 1002 and a band 1004. The front body 1002 is configured forplacement in front of eyes of a user in a reliable and comfortablemanner, and the band 1004 may be stretched to secure the front body 1002on the user's head. A display system 1080 may be disposed in the frontbody 1002 for presenting AR/VR imagery to the user. Sides 1006 of thefront body 1002 may be opaque or transparent.

In some embodiments, the front body 1002 includes locators 1008, aninertial measurement unit (IMU) 1010 for tracking acceleration of theHMD 1000, and position sensors 1012 for tracking position of the HMD1000. The locators 1008 are traced by an external imaging device of avirtual reality system, such that the virtual reality system can trackthe location and orientation of the entire HMD 1000. Informationgenerated by the IMU and the position sensors 1012 may be compared withthe position and orientation obtained by tracking the locators 1008, forimproved tracking of position and orientation of the HMD 1000. Accurateposition and orientation is important for presenting appropriate virtualscenery to the user as the latter moves and turns in 3D space.

The HMD 1000 may further include an eye tracking system 1014, whichdetermines orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes allows the HMD 1000 todetermine the gaze direction of the user and to adjust the imagegenerated by the display system 1080 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may also beused for real-time compensation of visual artifacts dependent on theangle of view and eye position. Furthermore, the determined vergence andgaze angles may be used for interaction with the user, highlightingobjects, bringing objects to the foreground, creating additional objectsor pointers, etc. An audio system may also be provided including e.g. aset of small speakers built into the front body 1002.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments andmodifications, in addition to those described herein, will be apparentto those of ordinary skill in the art from the foregoing description andaccompanying drawings. Thus, such other embodiments and modificationsare intended to fall within the scope of the present disclosure.Further, although the present disclosure has been described herein inthe context of a particular implementation in a particular environmentfor a particular purpose, those of ordinary skill in the art willrecognize that its usefulness is not limited thereto and that thepresent disclosure may be beneficially implemented in any number ofenvironments for any number of purposes. Accordingly, the claims setforth below should be construed in view of the full breadth and spiritof the present disclosure as described herein.

What is claimed is:
 1. An optical waveguide comprising: a plate oftransparent material comprising opposed first and second surfaces forguiding an optical beam therebetween by at least one of reflection ordiffraction; and a first surface-relief diffraction grating at the firstsurface for spreading the optical beam by diffracting portions thereofinto a non-zero diffraction order to propagate inside the plate; whereinthe first surface-relief diffraction grating comprises an array ofgrooves running parallel to one another and having a spatially varyingfill factor to provide a spatial variation of optical phase of theportions of the optical beam diffracted by the first surface-reliefdiffraction grating into the non-zero diffraction order, to reduceoptical interference between the diffracted portions of the opticalbeam.
 2. The optical waveguide of claim 1, wherein the grooves of thefirst surface-relief diffraction grating have a spatially varying widthto provide the spatially varying fill factor.
 3. The optical waveguideof claim 1, wherein the grooves of the first surface-relief diffractiongrating have a spatially varying thickness.
 4. The optical waveguide ofclaim 3, wherein the grooves of the first surface-relief diffractiongrating have a spatially varying width to provide the spatially varyingfill factor.
 5. The optical waveguide of claim 1, wherein the fillfactor is varying in a periodic pattern.
 6. The optical waveguide ofclaim 5, wherein a period of the periodic pattern is greater than 2 mm.7. The optical waveguide of claim 5, wherein the period of the periodicpattern is greater than 2 mm, and wherein the fill factor is varying ina sinusoidal pattern.
 8. The optical waveguide of claim 1, wherein thefill factor is varying in a pseudo-random pattern.
 9. The opticalwaveguide of claim 1, wherein the spatial variation of optical phase isno greater than 27c.
 10. The optical waveguide of claim 1, furthercomprising a second diffraction grating at the second surface foroutputting the optical beam by diffracting portions thereof to propagateout of the plate.
 11. The optical waveguide of claim 10, wherein thesecond diffraction grating is laterally offset from the firstsurface-relief diffraction grating in a direction of diffraction of theportions of the optical beam on the first surface-relief diffractiongrating.
 12. The optical waveguide of claim 10, wherein the seconddiffraction grating is disposed opposite the first surface-reliefdiffraction grating and comprises an array of grooves running parallelto one another and structured to provide a spatial variation of opticalphase of the portions of the optical beam diffracted by the seconddiffraction grating.
 13. The optical waveguide of claim 1, furthercomprising an input coupler for coupling the optical beam into theoptical waveguide.
 14. An optics block for a near-eye display, theoptics block comprising the waveguide of claim 13 and an image sourceoptically coupled to the input coupler for providing the optical beamthereto, wherein in operation, the optical beam carries an image to bedisplayed by the near-eye display.
 15. The optics block of claim 14,wherein the grooves of the first surface-relief diffraction grating havea spatially varying width to provide the spatially varying fill factor.16. The optics block of claim 14, further comprising a seconddiffraction grating at the second surface of the plate of transparentmaterial, wherein the second diffraction grating is configured foroutputting the optical beam by diffracting portions thereof to propagateout of the plate.
 17. A method for reducing a spatial variation ofthroughput of an optical waveguide comprising a plate of transparentmaterial having opposed first and second surfaces for guiding an opticalbeam therebetween by at least one of reflection or diffraction, themethod comprising: providing a first surface-relief diffraction gratingat the first surface of the plate, for spreading the optical beam bydiffracting portions thereof into a non-zero diffraction order topropagate inside the plate; wherein the first surface-relief diffractiongrating comprises an array of grooves running parallel to one anotherand having a spatially varying fill factor to provide a spatialvariation of optical phase of the portions of the optical beamdiffracted by the first surface-relief diffraction grating into thenon-zero diffraction order, to reduce optical interference between thediffracted portions of the optical beam.
 18. The method of claim 17,wherein the grooves of the first surface-relief diffraction grating havea spatially varying width to provide the spatially varying fill factor.19. The method of claim 18, wherein the fill factor is varying in aperiodic pattern.
 20. The method of claim 17, wherein the fill factor isvarying in a pseudo-random pattern.